This invention relates to a damping or absorbing element for absorbing the impact of pistons upon the ends of a power cylinder. More specifically, this invention relates to an impact damping element positioned on the ends of the cylinder or the facing surfaces of the piston for absorbing the impact of the piston against the cylinder end and preventing the piston from directly engaging the ends of the cylinder.
Generally, there are various types of impact elements used to dampen the impact of a piston against the ends of a cylinder at the end of each piston stroke in order to reduce the noise which may result from metal-to-metal contact as the piston engages the cylinder ends. U.S. Pat. No. 3,913,460, assigned to the assignee of the present invention, discloses a damping or cushioning piston impact element in the form of a contoured impact ring. The ring incorporates a resilient bumper portion and a resilient sealing lip. The sealing lip entraps the cylinder fluid between the piston and the cylinder end at the end of the piston stroke to provide an initial slowing of the piston as the piston approaches the end of the cylinder.
The main impact damping effects of the impact element described in the '460 patent are performed by the bumper portion of the impact ring. As the piston moves into close proximity to one of the cylinder end walls, the bumper portion is compressed between the piston and the cylinder end wall such that the bumper portion absorbs the energy of the impact. The bumper is designed such that under normal operating conditions, the piston will not engage the cylinder end.
As the velocity of the piston within the cylinder is increased or the weight of any element actuated by the piston is increased, resulting in increased kinetic energy, the load exerted on the bumper portion will also increase such that under certain extreme operating conditions, the bumper portion may compress sufficiently to permit metal-to-metal contact between the piston and the cylinder end. This contact between the piston and cylinder typically produces undesirable noise which the impact damping element is intended to prevent.
Conventional bumpers, such as the one described in the above referenced U.S. patent, are typically formed of a single homogeneous material such that the material characteristics of the sealing lip are the same as the material characteristics of the resilient bumper portion. Unfortunately, the material characteristics which are desirable for a sealing lip are not identical to those which are desirable for the bumper portion and a compromise must be made in selecting a material to perform both functions. For example, it is desirable to have a seal lip material which evidences low friction characteristics and a resistance to abrasion, and it is desirable to have a bumper portion material which evidences good characteristics for minimizing impact forces.
Accordingly, there is a need for an impact damping element which provides increased protection against contact between the piston and the cylinder end in the event that large loads are applied to the piston. In addition, there is a need for an impact element capable of absorbing increased loads and which retains the initial deceleration characteristics of prior art impact absorbing elements while also providing a sealing lip element which is resistant to abrasion.